RFID tags are known in the art. These so-called tags often assume the form factor of a label or a literal “tag” but are also sometimes integrated with a host article and/or its packaging. RFID tags typically comprise an integrated circuit and one or more antennas. The integrated circuit typically carries out a variety of functions including modulating and demodulating radio frequency signals, data storage, and data processing. Some integrated circuits are active or self-powered (in whole or in part) while others are passive, being completely dependent upon an external power source (such as an RFID tag reader) to support their occasional functionality.
There are proposals to utilize RFID tags to individually identify individual items. The Electronic Product Code (EPC) as managed by EPCGlobal, Inc. represents one such effort in these regards. EPC-based RFID tags each have a unique serial number to thereby uniquely identify each tag and, by association, each item correlated on a one-for-one basis with such tags. (The corresponding document entitled EPC Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz Version 1.0.9 is hereby fully incorporated herein by this reference.)
Each EPC-compliant tag has two states: “A” and “B.” The “A” state comprises the default state and hence represents the tag's state when initially powering up. Once a tag has been read its state changes from “A” to “B.” During the “A” state a tag will respond to any reader that offers a session query. During the “B” state the tag will not again respond to a reader using the same session query.
The EPC approach also supports four session protocols that differ with respect to how a read tag persists a “B” state. In Session “0” a read tag will persist this “B” state until power is lost and then the tag reverts immediately to the “A” state. In Session “1” a read tag will persist its “B” state for a period of time ranging from 500 ms to 5 seconds and will then automatically revert to the “A” state. In Session “2” and “3” a read tag will remain in the “B” state until power is lost. Then, once power is lost, the read tag will persist its “B” state for at least an additional 2 seconds (the actual persistence duration is left to the manufacturer and can reach minutes in some cases). In many cases a system designer will choose a particular EPC session protocol and employ that session protocol throughout a given facility.
In some cases a system designer will seek to provide more-or-less ubiquitous coverage through a given facility (such as a retail store) and thereby have the theoretical ability to read an RFID tag regardless of where that tag might be located within the facility. There are numerous (and significant) challenges, unfortunately, to designing and deploying such a system. On the one hand, continuously prompting all RFID tags to constantly (or even frequently) assume an A inventory state will typically result in those RFID tags constantly being read. When the facility includes a large number of RFID tags such a situation can result in an overwhelming number of reads that can literally prevent the system from providing useful data to the system user. Such a result can arise when the RFID-tag readers employ Session 0 or 1 as described above.
On the other hand, leaving read RFID tags in the B inventory state regardless of circumstance can leave the system blind to important in-facility events as pertain to those RFID tags. Such a result can arise when the RFID-tag readers employ Session 2 or 3 as described above.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. Certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. The terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.